This invention relates to thermoplastic films having static dissipative properties.
Antistatic agents have been used in the plastics industry for many years. In early usage, these agents functioned to reduce dust and dirt pick-up by plastic phonograph records, resin powders and plastic packaging materials, e.g., bottles and films. More recently, antistatic agents have the additional role of protecting electronic components which are marketed in thermoplastic film packaging. Without the use of a good antistatic system for a film package, an electronic component contained therein can be subjected to electrostatic discharge which can catastrophically destroy the component's efficacy or can create a latent defect in the component. These adverse effects are often seen when the electronic component is a solid state electronic device, such as computer chips which contain several semiconductive layers. Due to miniaturization and assignment of many functions to a single chip, the semiconductor layers are required to be very thin. Thus, even relatively small electrostatic discharges, say less than 500 V, can burn through the semiconductor layer and induce latent defects in or completely destroy the functionality of the semiconductor layer.
To reduce electrostatic buildup, the thermoplastic film industry has developed three types of antistatic treatments for such films, i.e., (1) applying an antistatic agent to the surface of the film, (2) rendering the film internally conductive, and (3) providing the film with an internal hydrophilic agent which gradually migrates to the film's surface.
An antistatic agent can be applied as a coat on the surface of the film by using dipping, spraying or wiping techniques. Exemplary of such antistats are Armak Company's ARMOSTAT 100C, 900, 910 and 920, which are quaternary ammonium compounds, and Ashland Chemical Company's VARSTAT K-22, which is an amine compound. The coating applied is conductive and thus is dissipative of the electrostatic charge. One drawback of surface treatment is that the coating can be worn or scuffed off, thereby yielding areas which can be susceptible to electrostatic charge buildup.
Rendering the film internally conductive is achieved, in non-cellular plastics, by incorporating, as an additive, a conductive material, e.g., conductive carbon, graphite fibers and carbon fibers, in the thermoplastic composition used to produce the film. The use of such additives is not a panacea though, as the additive may not be uniformly dispersed in the film or may adversely affect the desired physical properties of the film.
The use of an internal hydrophilic antistatic agent in the thermoplastic composition can give film produced therefrom an internal source of an antistatic agent. The agent gradually migrates from the interior of the film to its surfaces. Once at the film's surfaces, the hydrophilic nature of the antistatic agent causes a film of moisture to form on the surfaces and it is this film of moisture which provides the conductance needed to allow for electrostatic charge dissipation from the film. Since the antistatic agent is constantly migrating to the film's surface, a replenishable conductive surface is continually provided. These types of antistatic agents must, in general, be incompletely compatible with the plastic so that the migration phenomen is effected. Exemplary of such antistatic agents are quaternary ammonium compounds and ethoxylated phenolics. One factor weighing against using this treatment is that the formed film of moisture can adversely affect metals or other moisture sensitive items with which the film may come in contact.
Thus, there is a need for a novel thermoplastic film which has static dissipative properties but which does not suffer at all or at least to the same degree the limitations and difficulties delineated above for prior art thermoplastic films. It is therefore a object of the invention to provide such a thermoplastic film and a process for producing same.